Effects of interfacial adhesion on the rubber toughening of poly(vinyl chloride) Part 2. Low-speed tensile tests
Introduction
The toughness of a plastic can be enhanced substantially by blending with rubber particles [1]. Both morphological parameters and interfacial adhesion between the matrix and the dispersed particles play important roles in controlling the toughness of the blends. It is commonly believed that the effects of morphological parameters and interfacial adhesion are interrelated. For instance, strengthening the interfacial adhesion leads to a better dispersion of particles and smaller particle size and hence a change of toughness. This effect of interfacial adhesion on toughening is termed indirect [2]. On the other hand, interfacial adhesion may have a direct effect on toughening when the morphological parameters are identical [2].
At constant interfacial adhesion, the toughness of plastic–rubber blends measured at low speeds of deformation is a function of morphological parameters, i.e. rubber particle size, volume fraction and particle spatial distribution parameter. Muratoglu and coworkers [3] demonstrated that there is a substantial influence of both rubber particle size and volume fraction on the elongation-at-break and toughness of nylon 6-rubber blends. The toughness (defined as the area under the stress–strain curve to break) was found to increase with increasing rubber content for a constant rubber particle size and with decreasing rubber particle size for a constant rubber content. Takaki and coworkers [4] reported that the effects of both the size and volume fraction of methyl methacrylate–butadiene–styrene graft copolymer (MBS) particles on the elongation-at-break and the toughness of poly(vinyl chloride) (PVC)–MBS blends can be combined into the effect of a single parameter, namely the matrix ligament thickness. These properties were found to increase with decreasing matrix ligament thickness. Kim and coworkers [5] reported that the rubber particle spatial distribution has a substantial influence on the elongation-at-break values of styrene–acrylonitrile copolymer (SAN)–rubber blends.
However, interfacial adhesion has been reported to have little direct effect on the toughness of the polymer–rubber blends at low strain rates. Dompas and coworkers [6] investigated the effect of interfacial adhesion on the stress–strain curves of the PVC–MBS blends by using a constant volume fraction of MBS particles of size of 0.25 μm. They concluded that the interfacial adhesion has no influence on the stress–strain behavior of the blends. Huang and coworkers [7] found that interfacial adhesion has only a small effect on the fracture behavior of rubber-toughened epoxies. Cho and coworkers [8] investigated the effect of interfacial adhesion on the fracture toughness of the poly(methyl methacrylate) (PMMA)–rubber blends determined by three point-bending tests. By studying the fracture toughness as a function of the rubber particle size at a constant rubber content, they concluded that interfacial adhesion does not have any influence.
The interfacial adhesion between PVC and nitrile rubber (NBR) can be strengthened through an increase of the acrylonitrile (AN) level in NBR in the range of 0–40% by weight [9]. Imasawa and Matsuo [10] studied the strain rate dependence of the elongation-at-break for PVC–NBR blends with various AN contents. The NBR phase as well as the PVC phase in these blends were continuous [9]. However, it was not possible to separate the effects of morphological parameters and interfacial adhesion for blends with co-continuous phases due to lack of a theoretical framework.
In the first paper of this series, we reported the direct influence of interfacial adhesion on the impact strength of PVC–NBR blends with the morphology of well-dispersed rubber particles [2]. In the present work, we use the same PVC–NBR blends with two different levels of interfacial adhesion to investigate the direct effect of interfacial adhesion on the toughness of the blends determined by the low-speed tensile measurements. Microvoiding mechanisms, such as crazing [1], internal cavitation of rubber particles [6], [11] and debonding between the matrix and dispersed particles [2], [12], [13], [14], relieve the triaxial dilatational stresses ahead a crack tip, thereby promoting plastic deformation. Therefore, microvoiding is one of the most important toughening mechanisms. In this paper, the role of microvoiding in the toughening of the PVC–NBR blends is also discussed.
Section snippets
Materials and blend preparation
The raw materials used and the PVC–NBR18 and PVC–NBR26 blends were described in previous papers [2], [15].
Measurements of tensile mechanical properties
Tensile tests were performed according to ASTM standard D638M on an Instron 8511 machine at a crosshead speed of 100 mm min−1 and at a temperature of 25°C. The secant modulus at a strain of 0.02 was computed. The yield stress and elongation-at-break were also measured. The toughness was taken as the area under the stress–strain curve calculated with the aid of a computer.
Examinations of deformation mechanisms
Sectioning was
Morphology
All the PVC–NBR blends investigated have identical dispersion states of rubber particles, i.e. the morphology of well-dispersed particles shown in our previous papers [2], [15]. The sizes of the rubber particles in the blends follow the log-normal distribution. The average rubber particle sizes at the probability of 50% in log-normal plots are depicted in Fig. 1. The rubber particle size in the PVC–NBR blends increases generally with rubber volume fraction. At any given rubber volume fraction,
Conclusions
Using NBR rubber containing 18 and 26 wt% acrylonitrile we have prepared two types of blends, PVC–NBR18 and PVC–NBR26, with the PVC–NBR26 blends having stronger interfacial adhesion. This allows an investigation of the effect of interfacial adhesion on the tensile properties of the PVC–NBR blends with the morphology of well-dispersed NBR rubber particles.
At a low tensile speed of 100 mm min−1, the secant modulus and yield stress of the blends are independent of interfacial adhesion. On the other
Acknowledgements
Dr Zhehui Liu wishes to acknowledge the support of the Hong Kong Polytechnic University Postdoctoral Fellowship Scheme. We wish to thank the Hong Kong Research Grant Council and the NSFC (China) for financial supports.
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